If divers have not used both systems, what's the basis for an informed comparison? Heard someone comment on it? Read it on the internet maybe?
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I've dove both configurations. The only advantage backmounted, manifolded doubles provides in my expirience is that my backmounted collegues had an easier time getting on and off a boat. Thats it. In all other cases, in my expirience, side mount is the better option.
I agree with this to an extent - but it really depends on the boat and the SM system used.
In most cases, if you have room for double and a stage, you have room for side mount tanks and a stage, and you can bungee the tanks on the bench pretty much like a set of doubles.
However if it's a cattle boat intended for recreational divers only, with barely enough room for a tank and BC with no place for a second tank, it could get interesting, but most of the boats I have been on have had room for at least 2 tanks. In that case the only downside is walking a few feet with another tank and jumping off the boat. If you really only have room for a single tank, you can single tank in an AL 80 just fine.
On the boats where I have hot dropped in current, side mount works just as well as back mount, provided you use choker, hog rings or bolt snaps on the shoulders of the tanks to create a hard tank to D-ring connection to facilitate getting off and on the boat. SM or MB, the approach is the same - put everything on, stand on the stern rail and go in when you get the signal, You're still negatively buoyant and go right down. One SM advantage is if for some reason a dry suited BM diver goes in with his gas off and is very negative, he'll squeeze the suit to the point he can no longer reach the valves to turn the gas back on. In SM, that is not an issue.
In a small boat, you can easily take the tanks off, clip them to a gear line and come aboard, then pull the tanks up individually. That's a lot more difficult in BM.
1. In a sidemount configuration, any major gas loss from the first stage downstream loses you access to half of your gas. (Yes, you can close the valve and feather it, but it's not very comfortable or efficient.) That's true, but the odds of total gas loss are twice as long as the already long odds of that happening in backmount. Even if the odds of losing a reg and access to some of your gas is twice as great, in an all sidemount team losing access to one tank, still means you'll have half your gas left (which with proper management is enough, or if planned badly and/or unexpectedly delayed is still almost enough to get out) as well as all of your team mates reserve gas, which will cover any slack in your remaining gas due to a delay on exit post failure. *But see number 2...
2. Sidemount is much more idiosyncratic. It seems to take a lot more kit-fettling to get everything so it works. Backmount has fewer variables to play with -- tank height, wing height, and v- or tail-weight, that's about it. That's very true. I worry a bit about gas share protocols, etc, that may not be as mature or well thought out as they should be in the sidemount community. (Thank God for the RB guys though as they make us look good.)
*For example many SM divers just assume since the odds of losing all their gas are remote, they make no provisions fora long hose to share gas with a team mate, so if they push thirds and/or experience a significant delay during the exit after a failure at max p. and come up a few hundred psi short for the exit, life will get interesting - and swapping tanks is not really the answer. When you add in mixed teams with BM or RB divers, it gets even more interesting.
3. It CAN be more difficult to get two individual tanks to an entry site than to put two tanks in what operates as a backpack and walk them there. I have not found that to be the case. In fact, way back in the day when we'd have to climb down to some spots to dive, I dove independent doubles and attached the tanks with travel bands at the water. Also, with a Nomad and a tank rigging that allows you to bolt snap the tank to the shoulder D-rings, you can walk the tanks to the water just fine, and I think it's easier as your center of gravity is a lot lower, and if you do slip and fall, you're on top of the tanks, not under them. And if the going gets real rough I can carry one tank at a time on the harness. But that may not be the case with all SM harnesses.
4. Real estate for stages and deco bottles is a bit more limited. I have not found that to be the case. I can top clip a pair of stages, bottom clip another pair of stages (one travel/first deco gas or both bottom mix in one) and then still have room to accommodate a bottom clipped O2 bottle. I find it to be a lot cleaner than backmount and I don't have to resort to a leash or deal with tail high nose clipped tanks that add significant drag and potentially bang the hell out of a cave ceiling in smaller passages. If I need more than 7 bottles on a dive, I'll reconsider whether I should be doing it OC, and probably thumb the dive or scale back the plan.
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I do agree the profile is different, and it bothers me to see damage on the sides of caves that are due to SM divers not keeping enough distance from the walls, but that's a skills issue, or maybe an attitude issue. In reality, a sidemount diver can roll on his or her side and move through fissures with less damage than a BM diver as their profile is flatter top to bottom than an SM diver is side to side, so 90 degrees of roll, creates more room than a BM diver has when horizontal. Also, a sidemount diver can swing a tank forward and very quickly have a profile that would fit in the same horizontal box as a backmount diver.
And some of it is just subjective - and unfair. Caves have been damaged on floors and especially ceilings for years by BM divers in low passages and it's accepted as "normal" and nobody comments on the damaged cave ceilings in those low passages where BM divers had no business being due to the damage it causes - but sidemount divers apparently are not given the same latitude as it's not the "norm".
If there's a down side it is that the sidemount profile will mean different damage in some cases, but over the long term, I suspect we'll see less damage in total as more divers go sidemount and over time side mount will probably be the new norm.
I would imagine it could be harnessed to really be at the side, but maybe sloppiness is contagious at the BHB .
